Catheter and introducer catheter having torque transfer layer and method of manufacture

ABSTRACT

The instant invention relates generally to catheters and to introducer catheters used to help deliver catheters or other medical devices to locations within the human body. In particular, the instant invention relates to large diameter catheters and introducer catheters having a torque transfer layer that includes at least two flat wires braided into a wire mesh. The flat wires have a width of at least about 0.007 inches and a depth of at least about 0.003 inches. The lumen diameter of the catheter is at least about 6 French. The torque transfer layer provides increased strength, flexibility, and kink resistance.

BACKGROUND OF THE INVENTION

a. Field of the Invention

The instant invention relates generally to catheters and to introducercatheters used to help deliver catheters or other medical devices tolocations within the human body. In particular, the instant inventionrelates to catheters and introducer catheters having a torque transferlayer configured to provide increased strength, flexibility, and kinkresistance.

b. Background Art

Catheters are used for an ever growing number of medical procedures. Toname just a few examples, catheters are used for diagnostic,therapeutic, and ablative procedures. Typically, the physicianmanipulates the catheter through the patient's vasculature to theintended site, such as a site within the patient's heart. The cathetertypically carries one or more electrodes or other diagnostic ortherapeutic devices, which may be used for ablation, diagnosis, cardiacmapping, or the like.

It is known that, to facilitate placement of the diagnostic ortherapeutic catheter at a location of interest within the patient, itmay be introduced through another catheter, commonly known as a “guidingcatheter” or “introducer catheter,” and the terms will be usedinterchangeably herein. Generally speaking, an introducer catheter is atube having a high degree of directional control that is used to placeother catheters, which may have little or no directional control, intospecific areas of the patient's body.

In the field of cardiac ablation, for example, introducer catheters maybe used to negotiate the patient's vasculature such that an ablationdevice may be passed therethrough and positioned to ablatearrhythmia-causing cardiac tissue. The introducer catheter itself may beadvanced over a guide wire.

Generally, it is known that the introducer catheter must have an overalldiameter small enough to negotiate blood vessels while retaining aninner diameter (or “bore size”) large enough to accommodate the ablationdevice therethrough. Furthermore, since the path within the patient isoften long and tortuous, steering forces must be transmitted overrelatively great distances. Accordingly, it is desirable for theintroducer catheter to have sufficient axial strength to be pushedthrough the patient's vasculature via a force applied at its proximalend (“pushability”). It is also desirable for the introducer catheter totransmit a torque applied at the proximal end to the distal end(“torqueability”). An introducer catheter should also have sufficientflexibility to substantially conform to the patient's vasculature andyet resist kinking as it does so. One of ordinary skill in the art willrecognize that these various characteristics are often in tension withone another, with improvements in one requiring compromises in others.For example, increasing the bore size of an introducer catheter having agiven overall diameter requires utilizing a thinner wall. A thin-walledintroducer, however, is more likely to collapse upon itself when atorque is applied at its proximal end.

To improve pushability, torqueability, flexibility, and kink resistance,many extant introducer catheters utilize one or more reinforcing layersin their construction. For example, the guiding catheter disclosed inU.S. Pat. No. 4,817,613 to Jaraczewski et al. (“Jaraczewski”) includes apair of braided torque transmitting layers sandwiched between a flexibletubular member and a flexible plastic casing applied as a viscousmaterial and subsequently cured. Jaraczewski also teaches, however, thatto a certain degree, flexibility comes at the expense of torqueability.Further, depending on the thickness of the torque transfer layers, theymay increase the wall thickness, thereby either increasing the overalldiameter of the introducer catheter for a given bore size or decreasingthe bore size for a given overall diameter.

Many extant large bore introducers (i.e., an introducer catheter withbore size of greater than about 6 French), in order to find a suitablebalance of pushability, torqueability, flexibility, and kink resistance,have outer layers that are relatively stiff, which compromisestorqueability, kink resistance, and flexibility for pushability.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention provide a catheter, particularly anintroducer catheter, having good pushability, torqueability,flexibility, and kink resistance, while retaining a large bore size fora given overall diameter. In specific embodiments, an introducercatheter with good pushability, torqueability, flexibility, and kinkresistance has a bore size of at least about 6 French and an overalldiameter suitable for use in cardiac diagnostic and therapeuticprocedures. The invention further provides a method of constructing acatheter or an introducer catheter that does not require compromisingsome of the characteristics of pushability, torqueability, flexibility,and kink resistance for others of those characteristics.

In accordance with an aspect of the present invention, a catheter or anintroducer catheter for cardiac surgery comprises a tubular inner liner,a torque transfer layer, or reinforcing layer, surrounding at least aportion of the inner liner, the torque transfer layer comprising atleast two flat wires braided into a wire mesh, and an outer sheathformed over to the torque transfer layer. The flat wires aresubstantially rectangular in cross-section and have a width of at leastabout 0.007 inches and a depth of at least about 0.003 inches. Thetubular inner liner has a lumen diameter of at least about 6 French.

In specific embodiments, the catheter is an introducer catheter. Thetubular inner liner is polymeric and the outer sheath comprises amelt-processing polymer. The ratio of width to thickness of theintroducer catheter may be between about 2:1 and about 5:1. The torquetransfer layer has a braid density of between about 5 PPI and about 100PPI and may be braided in a one-over, one-under pattern, or a two-over,two-under pattern. The outer sheath comprises a plurality of segmentshaving differing hardness characteristics, and the segments are reflowbonded together.

In accordance with another aspect of the invention, a method ofmanufacturing a catheter of an introducer catheter for cardiac surgerycomprises forming a tubular inner liner about a mandrel, the linerhaving a proximal end, a distal end, a lumen diameter of at least about6 French; forming a torque transfer layer over at least a portion of theliner, the torque transfer layer comprising a plurality of braided flatwires, the flat wires being substantially rectangular in cross-sectionand having a width of at least about 0.007 inches and a depth of atleast about 0.003 inches: forming an outer sheath about the torquetransfer layer; and removing the mandrel. In specific embodiments, thetubular inner liner is polymeric and the outer sheath comprises amelt-processing polymer. The method further comprises heating the innerliner, torque transfer layer, and outer sheath to a temperaturesufficient to melt the melt-processing polymer.

The foregoing and other aspects, features, details, utilities, andadvantages of the present invention will be apparent from reading thefollowing description and claims, and from reviewing the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an introducer assembly in accordancewith one embodiment of the present invention.

FIG. 2 is a cross-sectional view of a steerable, large bore introducerin accordance with another embodiment of the present invention.

FIG. 3 depicts a reflow mandrel assembly used in the method ofmanufacturing introducers in accordance with the present invention.

FIG. 4 depicts an inner layer disposed over a reflow mandrel assembly inaccordance with a preferred method of manufacture.

FIG. 5 depicts a torque transfer layer disposed over an inner layer inaccordance with a preferred method of manufacture.

FIG. 6 depicts an outer sheath of varying components disposed over atorque transfer layer in accordance with a preferred method ofmanufacture.

FIG. 7 depicts the components of an introducer assembled over a reflowmandrel assembly having a distal configuration for a tip assembly.

FIG. 8 depicts a tip component, having a radiopaque marker, attached tothe distal end of the introducer depicted in FIG. 7.

FIG. 9 depicts another tip component, having a radiopaque marker,attached to the distal end of the introducer depicted in FIG. 7.

FIG. 10 is a perspective view of a steerable introducer in accordancewith an embodiment of the present invention, cut away to show details ofthe steering ring.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a catheter and a large bore introducercatheter suitable for use in the human vasculature for known medicalprocedures, such as cardiac ablation. For purposes of description, thepresent invention will be described in connection with a flat wireguided, or steerable, introducer catheter. It is contemplated, however,that the described features may be incorporated into any number ofcatheters or introducer catheters as would be appreciated by one ofordinary skill in the art. The large bore introducer catheter iscomprised of a combination of components and manufactured by either areflow process or an extrusion process, which provide the surprisingbenefits of allowing for introducer catheters having an internaldiameter of at least about 6 French while maintaining the desirableimproved properties of pushability, torquability, and flexibility, forouter diameters of sufficient size for navigation of cardiacvasculature.

FIG. 1 is a perspective view of an introducer assembly 110 according toone embodiment of the present invention comprising an introducercatheter 100 having a distal end 190 and a proximal end 104. Theintroducer catheter 100 may be operably connected to a handle assembly106 which assists in guiding or steering the introducer duringprocedures. The introducer assembly 110 further includes a hub 108operably connected to an inner lumen (not shown) within the handleassembly 106 for insertion or delivery of catheter assemblies, fluids,or any other devices known to those of ordinary skill in the art.Optionally, the introducer assembly 110 further includes a valve 112operably connected to the hub 108.

FIG. 2 depicts a cross-sectional view of an introducer catheter 200 inaccordance with one embodiment of the present invention. The introducercatheter 200 is comprised of a tubular polymeric inner liner 202, atorque transfer layer 204, an outer sheath 206 comprised of amelt-processing polymer, and a heat shrink layer 208. In the instancewhere the introducer is a steerable introducer, the introducer catheter200 further includes at least one flat wire 210 disposed longitudinallyalong the length of the introducer catheter 200. For purposes of thisinvention, a “flat wire” refers to a wire that is characterized by across-section that, when measured along two orthogonal axes, issubstantially flat. A flat wire typically has a rectangular crosssection, though the cross section need not be perfectly rectangular. Forexample, the present invention contemplates that a cross section of theflat wire may be oval, provided that the overall cross section isgenerally flat. As the term is used herein, a wire may be properlycharacterized as a flat wire if it has a cross section that is measuredx in one direction and at least 2× in a second direction generallyorthogonal to the first direction. A wire whose cross section issubstantially I-shaped may also be a flat wire if, generally, its heightis substantially greater than its width at its widest measurement. Oneof ordinary skill will appreciate that a flat wire may be defined in thecontext of the overall teachings of this application.

The at least one flat wire 210 may be further encased inside anotherpolymeric tubular member 212 forming a lumen 214 for housing the flatwire 210. The introducer catheter according to this embodiment ismanufactured by a reflow bonding process in which the components areindividually fed over a mandrel as discussed in more detail below.

The inner liner 202 is preferably a polymeric material, such aspolytetrafluoroethylene (PTFE) or etched PTFE. The inner liner 202 mayalso be made of other melt processing polymers, including, withoutlimitation, polyether block amides, nylon and other thermoplasticelastomers. Once such elastomer is Pebax® made by Arkema, Inc. Pebax ofvarious durometers may also be used, including without limitation, Pebax30D to Pebax 70D. In a preferred embodiment, the inner liner 202 is madeof a material with a melting temperature higher than the outer sheath206 such that the inner liner 202 will withstand the melt processing ofthe outer sheath 206.

Inner liner 202 defines a lumen 216 therethrough, preferably having adiameter 218 of at least about 6 French, more preferably of at leastabout 7 French, and most preferably of between about 10 French and about24 French. However, in some embodiments of the invention, it iscontemplated that lumen 216 may have a diameter 218 of up to about 32French or more, such as between about 7 French and about 32 French.

A torque transfer layer 204 is preferably disposed between the innerliner 202 and the heat shrink layer 208, more preferably between theouter sheath 206 and the inner liner 202. In the instance where theintroducer is a steerable introducer utilizing, for example, at leastone longitudinal wire 210, the torque transfer layer 204 may be disposedbetween either the inner layer 202 and the outer sheath 206 or the outersheath 206 and the heat shrink layer 208. The torque transfer layer 204may be made of stainless steel (304 or 316) wire or other acceptablematerials known to those of ordinary skill in the art.

The torque transfer layer 204 is preferably formed of a braided wireassembly comprised of flat wires, preferably stainless steel wiresincluding, for example, high tensile stainless steel wires. The torquetransfer layer 204 may be formed in any number of known braid patterns,including one-over-one (involving at least two wires) or two-over-two(involving at least four wires) crossover patterns. For lumen diametersof at least about 6 French, braided flat wires of at least about 0.003″thick by at least about 0.007″ wide, which heretofore were not used toform a wire mesh for the torque transfer layer, have producedsurprisingly good results of increased pushability, torqueability,flexibility, and kink resistance over non-flat wires and smaller flatwires. In general, the individual wires have a ratio of width to thethickness of at least about 2:1, including, for example, 2:1 to 5:1.Flat wires of about 0.004″ thick by about 0.012″ wide and of about0.004″ thick by about 0.020″ wide have also been braided with success toform torque transfer layers of superior performance.

The braid density, commonly measured in pixels per inch (“PPI”), istypically between about 5 and 100, and will depend on the size of theflat wires as well as the size of the catheter. For flat wires of atleast about 0.003″ thick by about 0.007″ wide and a catheter having aninner lumen of at least about 6 French, the PPI is preferably betweenabout 10 and about 90, more preferably between about 10 and about 55.For example, the PPI for flat wires of about 0.003″ thick by about0.007″ wide is preferably between about 20 and about 90, more preferablybetween about 35 and about 55 for an inner lumen of at least 6 French,and most preferably between about 35 and about 45 for an inner lumen ofat least about 10 French. The PPI for flat wires of about 0.004″ thickby about 0.012″ wide is preferably between about 15 and about 70, andmore preferably between about 15 and about 22 for all inner lumen of atleast about 6 French. The PPI for flat wires of about 0.004″ thick byabout 0.020″ wide is preferably between about 5 and about 50, and morepreferably between about 10 and about 20 for an inner lumen of at leastabout 6 French, and most preferably between about 10 and about 20 for aninner lumen of at least about 16 French.

Alternatively, the torque transfer layer 204 may utilize a varying braiddensity construction along the length of the introducer catheter 200.For example, the torque transfer layer may be characterized by a firstbraid density at the proximal end of the introducer catheter 200 andthen transition to one or more braid densities as the torque transferlayer 204 approaches the distal end of the introducer catheter 200; thebraid density of the distal end may be greater or less than the braiddensity at the proximal end. In a specific example, the braid density atthe proximal end is about 50 PPI and the braid density at the distal endis about 10 PPI. In another embodiment, the braid density at the distalend is about 20-35% of the braid density at the proximal end.

The torque transfer layer 204 may be formed separately on a disposablecore and subsequently slipped around the inner liner 202. One or moreportions of the torque transfer layer 204 may be heat tempered andcooled before incorporation into the introducer body 200 through methodsthat are known to those of ordinary skill. The action of heat temperingmay help to release the stress on the wire and help reduce radialforces. It is also contemplated that torque transfer layer 204 may bebraided directly on the inner liner 202.

A particularly preferred torque transfer layer 204 is comprised of0.003″ by 0.007″ 304 stainless steel wires at 35 PPI for an inner lumenof 6-10 French. Another preferred torque transfer layer 204 is comprisedof 0.004″ by 0.012″ 304 stainless steel wires at 22 PPI for an innerlumen of 12 French. Yet another preferred torque transfer layer 204 iscomprised of 0.004″ by 0.020″ 304 stainless steel wires at 13 PPI for aninner lumen of 16 French. These particularly preferred torque transferlayers may manufactured on a commercially available horizontal braidmachine set at 225 rpm utilizing a commercially available mandrel. Othersuitable methods of manufacturing the torque transfer layer 204 will beapparent to those of ordinary skill in the art.

The outer sheath 206 is preferably either an extruded Pebax or PTFEtubing. The melt-processing polymer of the outer sheath 206 occupies aplurality of voids of the wire mesh in the torque transfer layer. Theouter sheath 206 may also be made of other melt processing polymers,including, without limitation, etched PTFE, polyether block amides,nylon and other thermoplastic elastomers, at varying durometers. Theouter sheath 206 may also comprise more than one layer, including, forexample, two or more tubes of a melt processing polymer. Alternatively,as shown in FIG. 6, the outer sheath 306 may be comprised of varyingsegments 322, 324, 326, 328, 330 differing in hardness and/or materialalong the length of the introducer 300 and being reflow bonded together.This may be accomplished by layering or by placing annular rings ofdiffering materials along the length of the introducer 300. Varying thesheath composition in this manner provides the additional benefit ofadjusting flexibility, torquability, and pushability at various pointsalong the introducer 300.

In embodiments where the introducer is a steerable introducer (as shownin FIG. 2), at least one flat wire 210 is provided, preferably extendingalong substantially the entire length of the introducer. The flat wire210 is preferably composed of stainless steel and is preferably about0.002″×about 0.016″, and more preferably about 0.004″×about 0.012″ or0.016″. The flat wire may be selected such that the ratio of the widthto thickness is at least about 2:1. In one embodiment, at least aportion of the flat wire is encased inside a preformed tube 212 beforeplacement along the inner liner 202 to form a flat lumen 214. Thepreformed tube 212 need not be the same shape as the cross section ofthe flat wire, but instead, may be round, oval, rectangular, or anotherlike shape. Preferably, the preformed tube 212 has a cross section thatis not the same shape as a cross section of the flat wire 210, in orderto facilitate movement of the flat wire in the preformed tube. Thepreformed tube may be formed of PTFE, etched PTFE, polyether blockamides (such as Pebax), nylon, other thermoplastic elastomers, or anyother known material to one of ordinary skill in the art. Preferably,the preformed tube 212 has a higher melting point than the outer sheath206 so that the preformed tube 212 will not melt when the introducercatheter 200 is subjected to melt processing. In alternative embodimentsthe flat wire 210 may be covered with lubricious materials (not shown)before placement, including silicone and other lubricious materials.Alternatively, the flat wire 210 may also be coated with a lubriciouslayer to promote slideability, and it is also contemplated that the flatwire 210 may be manufactured with a smooth surface to promoteslideability. While stainless steel is a preferred material to composethe flat wire 210, other materials may be used, including, withoutlimitation, materials that are used for conventional round pull wires.More than one flat wire 210 may also be used, and in such cases, eachsuch flat wire 210 may be encased inside its own flexible tube 212.Preferably, as shown in FIG. 2, a pair of flat wires 210 are used thatare spaced at 180 degrees apart. The flat wires 210 are preferablyconnected to at least one steering ring 90 typically located near thedistal end of the introducer (see, e.g., FIG. 10). The proximal ends ofthe flat wires 210 are then operably connected to a steering mechanism(not shown) allowing for manipulation, or steering, of the introducercatheter 200 during use. FIG. 10 shows a cutaway view of the introducercatheter 200 according to the present invention highlighting thesteering ring 90 and its connection to the pull wires 210.

The basic method of manufacture according to an embodiment of thepresent invention will be described in reference to FIGS. 3-9. As thevarious components are assembled, the introducer components will becollectively referred to as an introducer. As depicted in FIGS. 3-9, amandrel 300, which is preferably round in cross-section and preferablyfrom about 6 inches to about 4 feet in length, is provided. As depictedin FIG. 3, the mandrel 300 has a distal end 350 and a proximal end 352.As depicted in FIG. 4, an inner liner 302 is placed on the mandrel 300.The inner liner 302 is fed on to the mandrel 300 and is then knotted onone end 320, or both ends.

As depicted in FIG. 5, a torque transfer layer 304 is then placed overthe inner liner 302. In the case of a steerable introducer catheter, theflat wire assembly (not shown) may then be placed over the torquetransfer layer 304. Alternatively, the flat wire assembly may be placedover an outer sheath 306. Another sheath layer (not shown) mayadditionally be placed over the flat wire assembly. The torque transferlayer terminates proximally of the distal end of the catheter.

Next, as depicted in FIG. 6, an outer sheath 306 is placed over thetorque transfer layer 304 and may be made of either single or multiplesections of tubing that are either butted together or overlapped witheach other. The multiple segments, or layers, of sheath material may beany length and/or hardness (durometer) allowing for flexibility ofdesign. FIG. 6 identifies a plurality of segments, 322, 324, 326, 328and 330. In this embodiment, the proximal end 330 of the outer sheath306 may be made of a material such as nylon, and the remainder of theintroducer may be made of one or more Pebax materials. The lengths ofthe various segments may vary, but preferably, the durometer hardnesslevels will decrease as the outer sheath 306 approaches its distal end.For example, a nylon base may then be followed by one or more of thefollowing Pebax segments: 70D Pebax; 60D Pebax; 55D Pebax; 40D Pebax;35D Pebax; 30D Pebax. The introducer shaft may also use one or moreblends of the foregoing Pebax materials, including, for example, 70D/60DPebax blend made by co-extrusion, or a 40D/35D Pebax blend made byco-extrusion. Preferably, the various components of the outer sheath 306according to this embodiment will be reflowed together duringmanufacturing. The proximal end of the shaft is preferably the longestsegment, and more distal segments may preferably vary between 0.25″ to6″, and more preferably from 0.25″ to about 3″. Preferably, the hardnesslevels of the segments and the lengths of the segments may be adjustedfor specific applications, and preferably, the distal end may have thelowest durometer levels of all segments. The shaft segments may beselected to improve flexibility, torquability, and pushability for thespecific application, as appreciated by one of ordinary skill in theart. Alternatively, the catheter may be formed by placing a thin innerjacket or layer (e.g., PTFE layer) onto a mandrel (e.g., stainless steelmandrel) or extruding a thin inner jacket or layer (e.g., Pebax layer)onto an extrusion mandrel (e.g., acetal mandrel), forming a torquetransfer layer over the inner layer, and extruding an outer jacket orsheath (e.g., Pebax jacket) over the torque transfer layer.

Lastly, a heat shrink layer 308 is placed over the assembled introducerassembly prior to reflow lamination. The heat shrink layer 308 ispreferably a fluoropolymer or polyolefin material, such as FEP, or othersuitable material as appreciated by one of ordinary skill in the art.

After assembly of the various components, the introducer assembly 300 issubjected to a reflow lamination process. FIG. 2 depicts a crosssectional view of the introducer assembly after this reflow process.Introducer assembly 200 may be laminated by heating the assembly untilthe material comprising the outer sheath 206 flows and redistributesaround the circumference. Preferably, the heat shrink layer 208 has ahigher melt temperature than the outer sheath 206, and during the meltprocess, the heat shrink layer 208 retains its tubular shape and forcesthe liquefied sheath layer material 206 into the torque transfer layer204 and into contact with the flat wires 210/preformed tubes 212 (ifpresent) and the inner liner 202. The introducer assembly 200 may thenbe cooled. The mandrel is preferably left in place during the coolingprocess as it helps the introducer assembly to retain its inner lumen ofat least about 6 French. The heat shrink layer 208 may be left on theintroducer assembly 200, or optionally removed. If the heat shrink layer208 is removed, the outer sheath 206 becomes the outside layer of theintroducer catheter 200.

Additionally, as shown in FIGS. 7-9, the present invention contemplatesthe inclusion of a tip assembly for use in medical procedures, such asan atraumatic tip, including, for example, a radiopaque materialcontained therein for location of the tip during use. For example, FIGS.7-9 depict a cross section of an introducer catheter 700 having a distalportion 730 configured to accept a tip assembly 732 or 734. In bothexamples, the tip 732 or 734 includes a ring 736, e.g., a radiopaquemarker, for location of the tip 732 or 734 during use. Additionally,FIG. 9 further includes a tip assembly 734 configured with a pluralityof port holes 738 for delivery of, for example, irrigation fluid. Thetip assembly may further be configured with ablation electrodes (notshown) operably connected to a power supply (not shown), for use incardiac ablation procedures.

Although several embodiments of this invention have been described abovewith a certain degree of particularity, those skilled in the art couldmake numerous alterations to the disclosed embodiments without departingfrom the spirit or scope of this invention. All directional references(e.g., upper, lower, upward, downward, left, right, leftward, rightward,top, bottom, above, below, vertical, horizontal, clockwise, andcounterclockwise) are only used for identification purposes to aid thereader's understanding of the present invention, and do not createlimitations, particularly as to the position, orientation, or use of theinvention. Joinder references (e.g., attached, coupled, connected, andthe like) are to be construed broadly and may include intermediatemembers between a connection of elements and relative movement betweenelements. As such, joinder references do not necessarily infer that twoelements are directly connected and in fixed relation to each other. Itis intended that all matter contained in the above description or shownin the accompanying drawings shall be interpreted as illustrative onlyand not limiting. Changes in detail or structure may be made withoutdeparting from the spirit of the invention as defined in the appendedclaims.

1. A catheter for cardiac surgery, comprising: a tubular inner linerhaving a lumen diameter of at least about 6 French; a torque transferlayer surrounding at least a portion of the inner liner, the torquetransfer layer comprising at least two flat wires braided into a wiremesh, the flat wires being substantially rectangular in cross-sectionand having a width of at least about 0.007 inches and a depth of atleast about 0.003 inches; an outer sheath formed over to the torquetransfer layer, wherein the inner liner and the outer sheath define acatheter wall; and a steering assembly comprising at least one pull wirethat is captured within the catheter wall.
 2. The catheter according toclaim 1, wherein the tubular inner liner is polymeric and the outersheath comprises a melt-processing polymer.
 3. The catheter according toclaim 2, wherein the melt-processing polymer occupies a plurality ofvoids of the wire mesh in the torque transfer layer.
 4. The catheteraccording to claim 1, wherein the lumen diameter is between about 7French and about 32 French.
 5. The catheter according to claim 1,wherein the at least two flat wires have a ratio of width to thicknessof at least about 2:1.
 6. The catheter according to claim 5, wherein theratio of width to thickness is between about 2:1 and about 5:1.
 7. Thecatheter according to claim 1, wherein the flat wires are about 0.004″thick by about 0.012″ wide.
 8. The catheter according to claim 1,wherein the flat wires are about 0.004″ thick by about 0.020″ wide. 9.The catheter according to claim 1, wherein the flat wires comprisestainless steel.
 10. The catheter according to claim 1, wherein thetorque transfer layer has a braid density between about 5 PPI and about100 PPI.
 11. The catheter according to claim 10, wherein the torquetransfer layer has a braid density between about 10 PPI and about 55PPI.
 12. The catheter according to claim 11, wherein the torque transferlayer has a braid density between about 35 PPI and about 45 PPI.
 13. Thecatheter according to claim 1, wherein the torque transfer layer isbraided in a one over, one-under pattern.
 14. The catheter according toclaim 1, wherein the torque transfer layer comprises at least four flatwires braided in a two-over, two-under pattern.
 15. The catheteraccording to claim 1, further comprising an atraumatic tip bonded to adistal end of the catheter.
 16. The catheter according to claim 1,wherein the outer sheath varies in hardness along its length.
 17. Thecatheter according to claim 16, wherein the outer sheath comprises aplurality of segments having differing hardness characteristics, whereinthe plurality of segments are reflow bonded together.
 18. The catheteraccording to claim 1, wherein the outer sheath includes a proximalsegment and a distal segment, the proximal segment having a greaterdurometer than the distal segment.
 19. The catheter according to claim18, wherein the distal segment comprises a tip segment and anintermediate segment positioned between the tip segment and the proximalsegment, and wherein the intermediate segment has a durometerintermediate to the tip segment and the proximal segment.
 20. Thecatheter according to claim 1, wherein the torque transfer layerterminates proximally of the distal end of the catheter.
 21. Thecatheter according to claim 1, wherein the outer sheath is reflow bondedto the torque transfer layer.
 22. The catheter according to claim 1,wherein the steering assembly comprises at least one flat pull wire. 23.The catheter according to claim 22, wherein the at least one flat pullwire is about 0.002″ thick by about 0.016″ wide.
 24. The catheteraccording to claim 22, wherein the at least one flat pull wire is about0.004″ thick by about 0.012″ wide.
 25. The catheter according to claim22, wherein the at least one flat pull wire is about 0.004″ thick byabout 0.012″ wide.
 26. The catheter according to claim 1, wherein thesteering assembly comprises two pull wires captured within the catheterwall and separated by about 180 degrees.